By Allen Houtz1 and Doug Cooper
override control using select elements in a previous article and learned
that environmental and energy efficiency concerns for
metered-air combustion processes can be partially addressed with a
select override element. Examples illustrated how a select override can either prevent
having too much fuel or too much air in the air/fuel mixture fed to
the burner of a combustion process, but one override element alone is not capable of preventing
In this article we explore the addition of a second select override
element to create a cross-limiting architecture that prevents the air/fuel
ratio fed to the burner from becoming overly rich (too much fuel) or lean (too much
air) as operating conditions change. Variations on this cross-limiting
architecture are widely employed within the air/fuel ratio logic of a broad range of
industrial combustion control systems.
Steam Boiler Process Example
To provide a larger context for this topic, we begin by
considering a multi-boiler steam generation process as shown below (click for a larger view):
Steam generation processes often have multiple boilers that feed a common steam
header. When steam is needed anywhere in the plant, the load is drawn from this common
header. Steam turbines, for example, drive generators, pumps and
compressors. Steam is widely used for process heating, can be injected into production vessels to
serve as a reactant or diluent, and may even be used to
draw a vacuum in a vessel via jet ejectors.
With so many uses, steam loads can vary significantly and unpredictably
over time in a plant. The individual boilers must generate and feed steam to
the common header at a rate that matches these steam load draws. Controlling
the steam header to a
constant pressure provides an important stabilizing influence
to plant-wide operation.
● Plant Master Controller
A popular multi-boiler architecture for maintaining header pressure is to use a
controller on the common header that outputs a firing demand signal for all of the boilers in the
steam plant. This steam header pressure controller is widely referred to as the
Based on the difference between the set point (SPP) and measured
pressure in the header, the Plant Master controller computes a firing demand
output that signals all of the boilers in the plant to increase or decrease
firing, and thus, steam production.
● Boiler Master
The Boiler Masters in the above multi-boiler process diagram are Auto/Manual selector stations with biasing (+/–)
values. If all three of
the Boiler Masters are in automatic, any change in the Plant Master output signal will pass through and create an associated
change in the firing demand for the three boilers.
If a Boiler Master is in automatic, that boiler is said to be operating
as a swing boiler. As such, its firing demand signal will vary (or
swing) directly as the Plant Master signal varies. If each of the fuel flow meters are scaled so that 100%
of fuel flow
produces maximum rated steam output, then each boiler will swing the same
amount as the Plant Master calls for variations in steam production.
But suppose Boiler B has cracked
refractory brick in the fire box or some other mechanical issue that,
until repaired, requires that it be operated no higher than, for example, 85% of its design steam production rate. That is, Boiler B has been derated and its maximum
permissible steam generating capacity has been lowered from the original
design rating. Two options we can consider include:
||When a Boiler Master
is in automatic, then: signal out = signal in + bias
the bias value is set by the
operator. If the bias value of Boiler Master B is set in this
example to –15%, then no matter what output is received from the
Plant Master (0% to 100%), the firing demand signal will never
exceed 85% (100% plus the negative 15% bias). In this mode of
operation, Boiler B will still swing with Boiler A and Boiler C in
response to the Plant Master, but it will operate at a firing rate
15% below the level of the other two boilers (assuming their bias
values are zero).
a boiler is suffering from refractory problems, then allowing the
firing rate to swing can accelerate refractory
degradation. Thus, Boiler Master B might alternatively be switched to
manual mode where the output firing demand
signal is set to a constant value. In manual mode, Boiler B is said to
base load of steam production. With the firing rate of
Boiler B set manually from the Boiler Master, it is unresponsive to
firing demand signal variations from the Plant Master. We then would have two swing boilers (Boiler A and Boiler C) and one base loaded
boiler (Boiler B).
Combustion Control Process
As shown below (click for a larger view),
each furnace and steam boiler has its own control system. Of
particular interest here is the maintenance of a specified air/fuel mass ratio
for efficient combustion at the burners.
As shown above, the air/fuel ratio control strategy receives a firing
demand from the Boiler Master. Air mass flow rate may be
measured downstream of the combustion zone and is thus shown as an input to
the ratio control strategy.
The boiler feed water and steam drum level controls
are not discussed here but can be found in this
3-Element Level Control
Ratio with Cross-Limiting Override Control
Certain assumption are used in the presentation that
||Air/fuel ratio is normally expressed
as a mass flow ratio of air to fuel.
The air and fuel flow transmitter signals are linear with respect to
the mass flow rate and have been scaled to range from 0-100%.
The flow transmitters have been carefully calibrated so that
both signals at the design air/fuel ratio are one to one. That is, if
the fuel flow transmitter signal, PVf, is 80%, then an air flow
of 80% will produce an air flow rate that meets the design air/fuel mass ratio. This
enables us to implement the ratio strategy without using multiplying relays
as discussed at the
end of this article.
Shown below (click for a larger view)
are the sensors, controllers, final control elements (FCEs) and function
blocks that might be included in the above dashed box labeled "ratio with
cross-limiting override control strategy."
Before discussing the details of the strategy, we rearrange the loop layout to make the symmetry of the design
more apparent (click for a larger view).
Specifically, we reverse the fuel flow direction (fuel now flows from right to left below) and show the air
mass flow rate transmitter as a generic measurement within the control
architecture. The control diagram above is otherwise identical to that
In any real process, different flow loops will have different
(the same change in controller output signal, CO, will produce a
different change in flow rate) and each loop itself will display a
nonlinear behavior over its range of operation (the process gain,
time constant and/or dead time will change as operating level changes).
The purpose of the
characterizing function block, f(x), is to
match the process gain of one loop over the range of operation with that
of the other loop. With matching signal-to-flow gains, this optional
function block simplifies the tuning of a ratio control strategy
with two flow control
loops. The characterizing function block, f(x), also
simplifies manual operation because the two flow CO signals will be approximately equal
at the design air/fuel ratio.
As shown above, the firing demand signal enters the high select override as a
candidate for the set point of the air flow controller (SPa).
In this cross-limiting strategy, the same firing demand signal enters the low
select override as a candidate for the set point of the fuel flow
As discussed in assumption 3 above, the flow transmitters have been
calibrated so that when both signals match, we are at the design air/fuel
mass flow ratio. Thus, because of the high select override, SPa
is always the greater of the the firing demand signal or the
value that matches the current fuel flow signal. And because of the low
select override, SPf is
always the lesser of the firing demand signal or the value that matches the current air
The result is that if firing demand moves up, the high select will pass
the firing demand signal through as SPa, causing the
air flow to increase. Because of the low select override, the fuel set
point, SPf, will not match the firing demand signal increase,
but rather, will follow the increasing air flow rate as it responds upward.
And if the firing demand moves down, the low select will pass the firing
through as SPf, causing the fuel flow
to decrease. Because of the high select override, the air set point, SPa,
will not match the firing demand signal
decrease, but rather, will track the decreasing fuel flow rate as it moves
In short, the control system ensures that during sudden operational changes
that move us in either direction from the
design air/fuel ratio, the burner will temporarily receive extra air until
balance is restored (we will be temporarily lean). While a lean air/fuel ratio
means we are heating extra air that then goes up and out the stack, it avoids the
environmentally harmful emission of carbon monoxide and unburned fuel.
Variable Air/Fuel Ratio
The basic cross-limiting strategy we have described to this point provides
no means for adjusting the air/fuel ratio. This may be necessary, for
example, if the
composition of our fuel changes, if the burner performance changes due to
corrosion or fouling, or if the operating characteristics of the
burner change as firing level changes.
Shown below (click for a larger view)
cross-limiting override control strategy that also automatically adjusts the air/fuel ratio
based on the oxygen level measured in the
As shown in the diagram, the signal from the air flow transmitter, PVraw,
is multiplied by the output of the analyzer controller, COO2,
and the product is forwarded as the measured air flow rate process variable,
With this construction, if the measured exhaust oxygen, PVO2,
matches the oxygen set point, SPO2,
then the analyzer controller (AC) output, COO2
will equal one and PVa
will equal PVraw.
But if the oxygen level in the
stack is too high, COO2 will become
greater than one. By multiplying the raw air flow signal, PVraw,
by a number greater than one, PVa
appears to read high. And if the oxygen level in the stack is too low, we
with a number smaller than one so that PVa
appears to read low.
The ratio strategy reacts based on the
artificial PV values, adjusting the air/fuel ratio until the measured oxygen
is at set point SP02.
This manipulation to the air/fuel ratio
based on measured exhaust oxygen is commonly called oxygen trim
control. By essentially changing the effective
calibration of the air flow transmitter to a new range, the signal ratio of
the carefully scaled air and fuel transmitters can remain 1:1.
Analyzers fail more often than
other components in the strategy, so when designing and tuning the analyzer controller, it is
important to limit how far COO2 can
move from its baseline value of one. Also, the analyzer
controller is effectively the primary (outer) controller in a
The secondary (inner) loop is the same air flow control loop being
driven by the Plant Master. As a result, it is advisable to tune the
oxygen (or combustibles) trim controller significantly more
conservatively than the Plant Master to minimize loop interactions.
Return to the
Table of Contents to learn more.
1. Allen D. Houtz
Automation Systems Group
P.O. Box 884
Kenai, AK 99611
Copyright © 2008 by Douglas J. Cooper. All Rights Reserved.